126 THORAX 



with the origins of the sterno-hyoid and sterno-thyreoid 

 muscles, which are attached to it. 



On the right, it is in relation with the upper part of the 

 mediastinal surface of the right pleura and lung (Fig. 57), 

 the right vagus nerve, and the arch of the azygos vein 

 (Fig. 13). It is also in relation, on its right side, near its 

 lower end and more anteriorly, with the superior vena cava, 

 and, at a higher level, with the innominate artery. 



Its left relations are the arch of the aorta, below, and 

 the left subclavian and left common carotid arteries, above. 



Bronchi. Each bronchus passes downwards and laterally, 

 first to the hilum of the corresponding lung, and thence 

 downwards in the substance of the lung to its lower end. 

 It can, therefore, be divided into an extra - pulmonary 

 and an intra - pulmonary portion. The extra - pulmonary 

 part, like the trachea, is kept permanently open by the 

 presence of curved cartilaginous bars in its walls; and 

 as the bars are deficient posteriorly, the extra-pulmonary 

 part of each bronchus presents a flattened posterior surface 

 similar to that of the trachea. The lumina of the intra- 

 pulmonary parts of the bronchi are kept patent by cartila- 

 ginous plates which are irregularly distributed in the substance 

 of the walls. 



Relations of the Extra -pulmonary Part of the Right 

 Bronchus.. The right bronchus is much more vertical than 

 the left (Fig. 62), and, as the ridge which separates the orifices 

 of the two bronchi at their origins lies to the left of the 

 median line of the trachea, the right bronchus is the 

 more direct continuation of the trachea, and foreign bodies 

 which have entered the windpipe pass more frequently into it 

 than into the left bronchus. It passes downwards and later- 

 ally from the upper border of the fifth thoracic vertebra to 

 the level of the upper part of the sixth thoracic vertebra, 

 where it enters the hilum of the right lung. Anterior to the 

 extra-pulmonary part of the right bronchus are the ascending 

 part of the aorta, the lower part of the superior vena cava, 

 and the right pulmonary artery. Above it is the arch of the 

 azygos vein ; and posterior to it are the azygos vein, the 

 posterior pulmonary plexus, and the right bronchial artery. 

 The extra-pulmonary part of the right bronchus gives off 

 one branch, which arises close to the hilum and is called the 

 eparterial bronchus, because it originates immediately above 



